My Mugen MBX-XR Works didnt come with a underdrive/overdrive gear set up at all, all the gears have the same teeth. The spur is 44T, the pinion are 40T Front/Rear, and the little spur gears in the diffs are 12T.

It sounds as though perhaps this buggy at some point had a front overdrive gear setup installed. This would make the front overdrive more than the rear(front diff spins more revolutions respective to the rear diff), you could fix this by simply counting and matching the front and rear diff teeth.

I would do this for starters, but you can always lock up the center diff if you like...... Although if the overdrive gear IS the problem, locking the center wont help at all.....NOR will locking the rear.....or the center AND the rear! Count the teeth buddy, and be done with it.